Many emerging viral pathogens are vertically transmitted in dipterans (dengue, West Nile, etc.). Understanding the genetic and evolutionary mechanisms that determine the virulence of vertically transmitted parasites is central to predicting and controlling these diseases. We will develop a model system combining theory with data for sigma virus, a vertically transmitted Rhabdovirus, in Drosophila melanogaster. The sigma virus-D. melanogaster system, with its ease of husbandry, extensive natural history, and wealth of genomic tools, is a superb model system for exploring 1) the evolution of virulence in general and 2) contrasting uniparentally vs. biparentally transmitted viruses in particular. Virulence, the damage done to the host, is a constantly evolving property in both established and emerging diseases; using an in vivo, animal model system to test the generality of results of virulence evolution from microbial or cell culture serial passage experiments is an essential initial step toward predicting and ultimately managing such evolutionary changes. Vertical transmission can maintain dangerous emerging viruses in their insect vector populations; information from a model system can help us predict and manage these viral reservoirs. We utilize a two-pronged experimental evolution study of sigma, an RNA virus, in intact animal hosts. First, we create replicated, artificial host shifts between D. melanogaster, in which infection is endemic; and in D. simulans, the host's sibling species, in which there is no infection. We thereby test predictions from microbial and cell culture serial passage experiments, which have not been evaluated before in an intact animal to the best of our knowledge. Specifically, we ask whether or not virulence necessarily increases with host shift; and whether or not shifted virus is necessarily attenuated (less virulent) in the original host. Next, we manipulate viremia via artificial selection in D. melanogaster, contrasting outcomes under biparental vs. uniparental transmission, to test whether or not rates of evolution of virulence are dependent on precise mode of vertical transmission as predicted by some theory. In both projects, we also explicitly quantify the relationship between viremia, virulence, and male transmission. We also characterize genomic outcomes and the source and nature of adaptations resulting from experimental evolution via whole-genome sequencing. The theoretical components of the project use a variety of modeling tools ranging from simple genetic models to simulations to understand conditions governing the rate of the evolution of virulence. We provide both a general theory aim, incorporating elements from evolutionary ecology and population genetics; and a second aim connecting data from this project with existing theory on the evolution of virulence. Our experiments and theory will provide a generalized, integrated understanding of evolution of virulence; and represent a significant step forward in our understanding of viral evolution as relevant to human health by testing results from existing theory and from microbial and cell culture in an intact, animal system. PUBLIC HEALTH RELEVANCE: This project aims to understand how vertically transmitted viruses (i.e., viruses transmitted from parents to offspring) become more or less harmful (i.e., change their virulence) and how they may change from one host to another. Data and models from this study will help scientists understand better how the virulence of human infectious diseases changes over time, how emerging diseases arise, and how we may be able to predict and control these changes; results will also help explain how insect-borne viruses such as West Nile and Dengue fever can persist in insect populations in the absence of human contact.